1. Field of the Invention
The present invention relates to a data slicing device and a data slicing method used for extracting either broadcast text data or TELETEXT signal that complies with the European standard, which are piggybacked onto a video signal, from the video signal.
2. Description of the Prior Art
Referring now to FIG. 8, there is illustrated a block diagram showing the structure of a prior art data slicing device. In the figure, reference numeral 1 denotes an A/D converter for converting a video signal into an equivalent digital signal at a rate n (n=2, 3, 4, . . . ) times as fast as the pulse repetition period of a clock run-in signal included in the video signal, 2 denotes a noise rejection circuit for rejecting noise components included in the digital signal obtained by the A/D converter 1, 3 denotes an amplifying circuit for detecting a variation in the digital signal or a transition of the digital signal whose noise components have been rejected or eliminated by the noise rejection circuit 2, and for amplifying the transition of the digital signal from the noise rejection circuit 2, 4 denotes a register for storing a reference value indicating a slice level, and 5 denotes a comparator for comparing the amplified transition of the digital signal from the amplifying circuit 3 with the reference value stored in the register 4 so as to extract data from the digital signal. FIG. 9 illustrates the waveform of a horizontal synchronizing signal for the video signal onto which data are piggybacked.
In operation, when the A/D converter 1 receives the video signal, it converts the video signal into an equivalent digital signal at a rate n (n=2, 3, 4, . . . ) times as fast as the pulse repetition period of a clock run-in signal included in the video signal. After the A/D converter 1 converts the video signal into an equivalent digital signal, the noise rejection circuit 2 eliminates noise components from the digital signal by averaging the values of any (n/2) or less pulses included in the digital signal. For example, when n=8 and the noise rejection circuit 2 averages any four successive pulses included in the digital signal, the noise rejection circuit 2 obtains the sum of the values of any four successive pulses included in the digital signal first, and then divides the sum by 4 to carry out averaging of the digital signal.
When the noise rejection circuit 2 eliminates noise components from the digital signal obtained by the A/D converter 1, the amplitude of the digital signal is reduced. The amplifying circuit 3 then differentiates the digital signal from the noise rejection circuit 2 so as to detect a transition of the digital signal. In other words, the amplifying circuit 3 obtains a difference between the values of a preceding pulse and a current pulse of the digital signal. The amplifying circuit 3 then adds the difference to the later one of the two digital values, i.e. the current digital value so as to amplify the amplitude of the digital signal from the noise rejection circuit 2. As a result, the difference between the preceding and current values of the digital signal is amplified and the transition of the digital signal is therefore enhanced.
After the amplifying circuit 3 amplifies the digital signal, the comparator 5 compares the current value of the digital signal from the amplifying circuit 3 with the reference value stored in the register 4 so as to extract data from the digital signal. If the value of the current pulse of the digital signal is greater than the reference value, the comparator 5 furnishes an output signal at a HIGH level for the pulse. Otherwise, the comparator 5 furnishes an output signal at a LOW level for the pulse.
While the prior art data slicing device which is so constructed as mentioned above can extract data piggybacked onto a video signal from the video signal, it has a problem in that when the amplitude of the video signal is low or when data signals of different peak-to-peak amplitudes are piggybacked onto the video signal, it cannot extract data precisely because the reference value stored in the register 4 does not always lie in the middle range of the values of all data to be extracted, i.e. in a range intermediate between the maximum and minimum amplitudes or values of the digital signal.
Except when data signals of different peak-to-peak amplitudes are piggybacked onto the video signal, the amplitude of the video signal is constant. However, if noise is piggybacked onto the video signal, the amplitude of a dc component included in the video signal is varied and hence the middle range of the values of data extracted from the video signal is varied. As a result, the reference value stored in the register 4 deviates from the middle range of the values of data to be extracted from the video signal. That's because it is difficult for the noise rejection circuit 2 to eliminate noise components from the video signal completely.